Triethanolamine modifier and the waterborne epoxy resin were applied for the surface modification treatment of the expanded polystyrene(EPS) grain. By means of scanning electron microscopy（SEM）, the mechanism of interfacial modification of EPS cement based composite materials was investigated.The results were shown that the adhesion strength between EPS grain and cementing material can be improved by interaction between the triethanolamine and polymer. The core shell structure constructed by the modified EPS grain and inorganic gelling material can improve the mechanical properties of EPS cement based composite materials.